In what will be one of the most high-profile political set pieces in Washington in weeks, John Kerry and Chuck Hagel will testify to the Senate Foreign Relations committee.

America's top military officer, General Martin Dempsey, chairman of the Joint Chiefs of Staff, will also go before the panel.

Kerry will argue that failing to act in Syria "unravels the deterrent impact of the international norm against chemical weapons use," a senior State Department official said on condition of anonymity.

Inaction also "endangers our friends and our partners along Syria's borders... and risks emboldening Assad and his key allies - Hezbollah and Iran," the official warned.

As the White House battles for congressional authorization to bomb Syria, two top Republicans warned Monday that a "no" vote after President Barack Obama had threatened action would be catastrophic.

Hawkish senators John McCain and Lindsey Graham emerged from an hour of talks with Obama suggesting that the White House could be mulling a wider military campaign in Syria than first thought, along with more robust support for the opposition.

Obama shocked Washington and the world on Saturday when he decided to seek support for military action in Syria from Congress, when it seemed US cruise missile strikes on President Bashar al-Assad's forces and assets were imminent.

McCain and Graham appeared to offer qualified backing for Obama's plans.

"A vote against that resolution by Congress I think would be catastrophic," said McCain.

"It would undermine the credibility of the United States of America and the president of the United States. None of us want that."

Graham warned of the wider consequences of a failure to back military action.

"I can't sell another Iraq or Afghanistan, because I don't want to," Graham said.

"(What) I can sell ... (is) that if we don't get Syria right, Iran is surely going to take the signals that we don't care about the nuclear program."

The senators also hinted at the administration's evolving strategy for Syria.

Obama has stressed that any US action, expected to include cruise missile attacks, would be "limited" and "narrow."

But McCain said he had "been given some reason to believe that very serious strikes may take place as opposed to cosmetic (ones)."

"I don't think it is an accident that the aircraft carrier is being moved over in the region," he said.

ABC News reported Monday that the USS Nimitz aircraft carrier was moving westward toward the Red Sea, though had not yet received orders to support a strike on Syria.